Method of casting metals in metal mold

ABSTRACT

A process for casting an exotic alloy in a metal mold comprising a relatively easily machined steel mold wherein the mold is prefabricated from component parts having those surfaces which are exposed to the cast metal being machined and finished by polishing. The mold formed is characterized by the unusually complex forms that are possible and is preferably removed partially by rough machining and the remainder being dissolved by an acid which will not attack the cast metal. The casting is recovered in a precise highly finished form requiring no further finishing process.

United States Patent 1 Frederick I 51 Feb. 26, 1974 METHOD OF CASTING METALS IN METAL MOLD [76] Inventor: Jean Robert Frederick, 3130 State Rd., Ashtabular, Ohio 44004 221 Filed: May 17, 1971 [21] App]. No.: 143,997

[52] US. Cl 164/68, 164/80, 164/131 [51] Int. Cl... B2211 21/02, B22d 23/06, B22d 29/00 [58] Field of Search 164/68, 80, 131, 132, 137;

[56] References Cited UNITED STATES PATENTS 2,787,536 4/1957 Spedding et a1. 164/68 X 3,473,599 10/1969 Rose 164/132 X 1,544,929 7/1925 Pack 164/132 X 2,609,576 9/1952 Roush et a1 [64/132 X 3,148,237 9/1964 Pennington et a1 75/171 X Primary Examiner-Robert D. Baldwin Attorney, Agent, or FirmCennamo, Kremblas &

Foster [5 7] ABSTRACT 5 Claims, 4 Drawing Figures PATENT'EB E826 I974 FIG?) FIGI INVENTOR JEAN ROBERT FREDERICK enuamo Kenn 15 6" jualer ATTORNEYS BACKGROUND OF INVENTION The present invention relates generally to an investment type casting process and particularly to a novel improved process for casting very accurate and particularly intricate metal castings in a metal mold.

Many prior art casting processes are well-known and quite suitable for producing relatively non-intricate castings. However, all of these methods leave much to be desired when intricate articles of very accurate detail are to be made by casting. Electrical discharge machines are presently used to create intricate articles. However, this process is very expensive and has some limitations which are not factors in a casting process. Further, articles composed of extremely hard metals, which are difficult to machine, or articles having complex shapes which require a high degree of accuracy in detail may be cast using the method of the present invention.

SUMMARYOE INVENTION In general, the method of the present invention comprises constructing a metal master mold having a cavity of intricate shape and predetermined design using machined matching parts which are assembled and suitably fastened together. This master mold consists of a predetermined metal having a higher melting point than the metal which is to fonn the casting. The metal chosen for the casting must also be resistant to a chemical agent which will readily dissolve the metal used for the master mold. The master mold must be constructed in a leakproof manner to prevent the molten casting metal from running through the mold and becoming lost.

The master mold may include, in addition to the cavity for the casting, the gates, sprue and the ladle. The ladle portion is loaded with the casting metal and the whole unit is heated to a temperature which will liquify the casting metal. This is preferably done in a protective atmosphere. The time and temperature are closely controlled according to the given materials employed. After the casting material has filled the cavity and cooled, the master mold material is largely machined away. The rough remainder is then placed in a predetermined chemical agent which dissolves the remaining master mold material but which will not attack the casting material. The result is a virtually finished, one piece casting which is extremely accurate in intricate detail.

OBJECTS OF INVENTION It is therefore a primary object of the present invention to provide a method for producing metal castings that are cast in a virtually finished condition more economically than similar shapes of the same material can be produced by prior art methods.

It is another object of the present invention to provide a method of the type described which permits a cast article to be produced which has extreme accuracy in detail, typically in the order of approximately 0.001 inches per inch or less.

It is another object of the present invention to provide a method of the type described which permits articles of unusually intricate design to be cast of a metal having the desired characteristics for the finished product but which characteristics and intricate design render conventional techniques extremely difficult, too expensive, or impractical.

Further objects and advantages of the present invention will be apparent from the following description, references being made to the accompanying drawings wherein a preferred form of an embodiment of the invention is clearly shown.

IN THE DRAWINGS FIG. 1 is a side elevational diagrammatical view in section of a casting apparatus constructed to practice tbe method of the present invention;

FIG. 2 is a side elevational view illustrating a step of the method of the present invention diagrammatically illustrating the mold disposed in a chemical bath;

FIG. 3 is a side elevational view similar to the view shown in FIG. 2 illustrating the finished casting after the mold has been dissolved in the chemical bath; and

FIG. 4 is a partial side sectional view of a ladle, sprue and gate construction for casting apparatus which may be used in the method of the present invention illustrating an arrangement which permits gases to escape the cavity to avoid formation of undesirable voids in the finished casting.

DETAILED DESCRIPTION OF INVENTION Referring in detail to the drawings, a master mold indicated generally at 20, and constructed in accordance with the present invention is diagrammatically illustrated in FIG. 1 and includes a ladle 22, sprue 24, gate 26, and the cavity 28 which is the negative of the article to be cast.

A heat shield 30 may be provided, if necessary, to protect the cavity section from overheating. When a shield is used, it should be removed after melting of the casting material is accomplished to allow cooling in a prescribed manner.

One of the primary considerations in using the method of the present invention is the choice of the metals used for constructing the master mold and the casting. The metal of the casting must have a lower melting point than that used to construct the master mold and further must be resistant to a chemical agent which will readily dissolve the metal used for the master.

In addition, the metal used for constructing the master should be relatively easily machined as it is highly desirable to construct the pattern by developing the intricate shapes using sectional matching parts. These parts may then be fastened together by screws, pins or rivets for example, which are made of the same metal as that of the master.

By constructing the master mold in this manner, it is much easier to obtain complex forms and therefore fashion the cavity in extremely intricate shapes ordinarily very difficult, if not impossible to produce in an integral form. An additional advantage obtained is that protrusions are usually more readily formed, if complicated, than indentations identical in shape. In comparsion to sinking a die cavity of complex shape in a solid material, the above-described procedure for creating die cavities is relatively simple.

Further, by using machined sectional parts to fabricate the master mold, extreme accuracy in detail may be obtained exceeding the accuracy obtainable using prior art casting methods and means.

Of course, using the casting method of the present invention, articles composed of metals which are extremely difficult to machine can be cast with accuracy in detail commensuate with a machined article.

The method of the present invention permits articles to be cast that heretofore could only be practically formed using an electrical discharge machine and further permits articles to be formed that would be very difficult, if not totally impractical even using that process.

As one example of the method of the present invention, a master mold 20 was constructed using a mild low carbon steel which may be readily machined in sections and assembled in the form shown in FIG. 1. Although master 20 was actually fabricated in many parts and fastened together, in the interest of brevity, the details are not shown, as once given the desired form a skilled workman could readily reproduce such a structure. The ladle section 22 was loaded with a cobaltchromium-tungsten alloy and the whole unit was then placed in a furnace and heated slightly above the melting point, 2327 Farenheit, of the alloy. This temperature was held substantially constant for fifteen minutes to allow the molten alloy to fill the cavity. The melting point of the steel used for the mold was 2786 Farenheit.

Using this particular combination of materials, a protective atmosphere of hydrogen was employed within the furnace to prevent oxidation of the mild steel mold. In some applications other gases, such as helium, argon or nitrogen may be used for example, to prevent oxidation.

It is important to point out that in this example, as in other applications, the temperatures involved are critical in that alloying of the steel pattern and the casting metal takes place. The higher the temperature the greater the alloying effect. This alloying effect must be controlled to maintain the high degree of accuracy possible using the teaching of the present invention and therefore precise control of the temperature-time values is very important. In this particular example, the casting alloy will destroy the steel pattern at a temperature of 2450 Farenheit.

A temperature excess of merely 20 above the alloy melting point held for minutes was observed to increase surface alloying between 0.002 and 0.003 inches. Therefore it should be readily appreciated that in most applications the time-temperature values employed are extremely important to maintain the desired accuracy of the final cast article.

After appropriate heating of the mold and the casting metal, the shield is removed and the unit is permitted to cool. After cooling, gate 26 is ground through separating the cavity 26 from the remainder of the pattern and a major portion of the steel mold surrounding cavity 26 is machined away leaving the remainder illustrated in dotted lines 32 in FIG. 1. This remainder portion of the steel mold is immersed in a nitric acid solution and dissolved leaving only the finished casting 34 with at most only a very minor protrusion of the gate 26 still to be removed.

In this example, the finished casting is a one piece, hard, heat and corrosion resistant article which is extremely accurate in detail deviating from the size and. shape of the cavity by only a matter of approximately 0.001 inch per inch.

It is important to point out that it is possible to produce metal castings which have small holes extending both laterally and longitudinally, irregular ducts, and virtually innumerable shapes and designs very difficult or even impossible to form using prior art techniques.

The method of the present invention is particularly applicable in the formation of die-casting dies, molds for plastics, heat and corrosion resistant jigs and fixtures, complex nozzles and other intricate articles from metals heretofore not readily formable using prior art techniques, and to specification limits which could not heretofore be cast.

It is important to point out that other metals may be used as well as variations in standard casting practices such as, for example, heating methods and methods of pouring the casting material without departing from the spirit of the present invention. The specific protective atmosphere, the chemical agents and the choice of materials may vary within the concept of the present invention according to the end result desired. Further, protective coatings may be employed on the surface of cavity 26 to inhibit the alloying effect, if desired.

Further conventional foundry practices may be employed with respect to the form of the ladle, sprue, and gates without departing from the spirit of the present invention. For example, FIG. 4 illustrates a construction of a vent 36 to prevent entrapment of gases which may cause defects in the cast part. Further, it should be noted that the larger mass of mold 20 centers around the area of sprue 24 and gate 26. This is designed so that the last place to cool will be in a position which will be discarded since shrinkage defects may appear in the last part of the casting to solidify. By suitable design, the void caused by shrinkage upon cooling is then located outside of cavity 28.

I claim:

1. A method for casting a metal in a metal mold comprising the steps of filling the cavity of a metal mold with a molten metal having a melting point greater than 2000 F but lower than the metal of the mold; controlling the temperature of the mold and the molten cast metal between the melting point of the cast metal and 20 F above said melting point for a predetermined time interval; controlling the cooling rate of the cast metal in the mold to obtain a slower cooling rate in that portion of the casting adjacent to the sprue until complete solidification is achieved; machining a substantial portion of the metal mold away from the solidified cast metal; and removing the remainder of the metal mold by dissolution in a chemical agent which is inert with the cast metal.

2. A method for casting a metal in a mild steel mold comprising the step of filling the cavity of said mold with a cast metal comprising an exotic alloy having a melting point greater than 2000 F but lower than the steel mold by heating said mold and the cast metal simultaneously to a predetermined temperature above the melting point of the cast metal, said temperature being related to minimizing the alloying reaction between the cast metal and the metal mold; cooling the mold and the cast metal to cause solidification of the casting; and removing at least a substantial portion of the metal mold by dissolution in a chemical agent which will not effect the metal of the casting.

3. The method defined in claim 2 including heating the mold and the cast metal in an atmosphere which inhibits oxidation.

4. The method defined in claim 2 wherein said mold 5. The method defined in claim 2 wherein said cast cavity is constructed of a plurality of machined matchmaterial is a metal alloy including cobalt, chromium ing metal portions having each of the surfaces of the and tungsten. formed cavity being finished by polishing. 

1. A method for casting a metal in a metal mold comprising the steps of filling the cavity of a metal mold with a molten metal having a melting point greater than 2000* F but lower than the metal of the mold; controlling the temperature of the mold and the molten cast metal between the melting point of the cast metal and 20* F above said melting point for a predetermined time interval; controlling the cooling rate of the cast metal in the mold to obtain a slower cooling rate in that portion of the casting adjacent to the sprue until complete solidification is achieved; machining a substantial portion of the metal mold away from the solidified cast metal; and removing the remainder of the metal mold by dissolution in a chemical agent which is inert with the cast metal.
 2. A method for casting a metal in a mild steel mold comprising the step of filling the cavity of said mold with a cast metal comprising an exotic alloy having a melting point greater than 2000* F but lower than the steel mold by heating said mold and the cast metal simultaneously to a predetermined temperature above the melting point of the cast metal, said temperature being related to minimizing the alloying reaction between the cast metal and the metal mold; cooling the mold and the cast metal to cause solidification of the casting; and removing at least a substantial portion of the metal mold by dissolution in a chemical agent which will not effect the metal of the casting.
 3. The method defined in claim 2 including heating the mold and the cast metal in an atmosphere which inhibits oxidation.
 4. The method defined in claim 2 wherein said mold cavity is constructed of a plurality of machined matching metal portions having each of the surfaces of the formed cavity being finished by polishing.
 5. The method defined in claim 2 wherein said cast material is a metal alloy including cobalt, chromium and tungsten. 